(a) Field of the Invention
The present invention relates to an optical apparatus which deflection-scans light emitted from a light source using a rotary polygon mirror and reflects the light by a recording material and irradiates the same, and the invention also relates to an image forming apparatus such as an electrophotographic copying machine and a printer.
(b) Description of the Related Art
A recent electrophotographic type copying machine or printer converts image information into a digital signal, irradiates a photosensitive drum charged by a charger by means of a scanning type optical apparatus with light based on the signal, thereby forming an electrostatic latent image, and forms the electrostatic latent image into a visible image as a toner image by a development unit.
The scanning type optical apparatus deflection scans light emitted from a light source by a rotary polygon mirror which is a deflection-scanning means, reflects the light by a reflection mirror, thereby irradiates a photosensitive drum disposed at a predetermined position with the light. At that time, in the case of the scanning type optical apparatus in which the reflection mirror is disposed in the vicinity of the photosensitive drum, the optical apparatus is influenced by surface precision of a surface of the reflection mirror, and a curve is generated in a scanning line on the photosensitive drum.
In the case of a system which scans laser beam emitted from the same laser light-emitting source on a single photosensitive drum, for both monochrome image and a color image, even if a slight curve is generated in the scanning line, since the scanning is carried out with the same characteristics, deviation is not generated on the photosensitive drum, and a problem that this curve deteriorates an image quality is not caused so much.
However, in the case of an image forming apparatus which scans a plurality of drums to form a color image, since light passes through different optical elements and is reflected and different photosensitive drums are irradiated with the light, each color has different curved profile of the scanning line. In that case, when colors are superposed on an image, the scanning lines are not superposed and colors are deviated, and this deteriorates the image quality.
FIG. 6(a) shows one example of the color deviation. A lateral axis shows image heights of yellow (Y), magenta (M), cyan (C), black (Bk) which form a color image, and a vertical axis shows an irradiation position on each drum. When the scanning line is not curved, the light source has a straight profile. However, in the case of the scanning type optical apparatus which scans a plurality of photosensitive drums, a degree of the curve and profile are varied as described above and as shown in the drawing, and the color deviation is generated.
Therefore, the scanning type optical apparatus is conventionally provided therein with means for correcting the curve of the scanning line. As shown in FIG. 6(b), there is used a technique for adjust the colors such that in accordance with one of colors having greatest curving amount, other three colors are adjusted so that the scanning lines of all of the four colors are superposed. The following conventional correcting method of a scanning line curve has been proposed.
Japanese Patent Application Laid-open No. H10-268217 describes a method in which a curve adjusting method bends a plastic lens, thereby the curve of the scanning line. According to the method using the plastic lens however, if the temperature of the optical apparatus rises, the optical characteristics of the plastic lens are varied, and the correcting degree of the curve of the scanning line is varied and this is not preferable.
There is another method for correcting the color deviation by curving a reflection surface of a recording material having small variation in external factor.
Japanese Patent Application Laid-open No. H8-146325 describes a structure in which opposite ends of a flat reflection mirror are pushed by springs, the mirror is curved by variable pressing means which is provided on a central portion of the mirror, and the curve of the scanning line is corrected. Japanese Patent Application Laid-open No. 2001-117040 describes a structure in which reflection surface sides of a mirror are supported by a pair of brackets, a back side of a portion of the mirror projected outward from the bracket is supported by an adjustment screw, and the mirror is curved by the adjustment screw.
In a conventional scanning type optical apparatus using the technique which corrects a curve of the scanning line by curving the reflection r, an optical box in which an optic such as the reflection mirror is accommodated is provided with a seat surface for supporting the reflection mirror. A shape of a portion of the seat surface which comes into contact with the reflection mirror and supports the same is flat in shape as shown in FIG. 7(a).
FIG. 7(a) is an enlarged view of a portion of the optical box on which the mirror is placed. The reflection angle of the mirror is defined by abutting the mirror against seat surfaces 70, 71 and 72, and a position of the mirror in its longitudinal direction is defined by abutting an end of the mirror in the longitudinal direction against an abutment portion 73.
FIG. 7(b) is an enlarged view of the seat surface which supports the reflection mirror to be curved. A curved mirror 74 is supported by seat surfaces 70 and 71 of an optical box and by a spring 75 which pushes the mirror 74 against the seat surfaces 70 and 71. FIG. 7(c) is a further enlarged view of an edge (portion surrounded by broken line in FIG. 7(b)) formed in the optical box. As can be found also in FIG. 7(c), when the mirror is curved, angle portions of the seat surfaces 70 and 71 formed in the optical box interfere with the ideal curved shape of the mirror in a region shown with hatching.
As described above, the seat surface portion and the reflection mirror interfere with each other depending upon the shape of the seat surface in a direction in which the reflection mirror is curved and with this, the mirror is newly deformed locally in the vicinity of the seat surface, and there is an adverse possibility that the optical characteristics of the mirror are deteriorated from the support portion of the reflection mirror toward the reflection surface.
That is, this is because that if the seat surface is of flat shape while the reflection mirror is curved, interference with the edge by the flat shape of the seat surface is generated.
Further, if the receiving portion of the reflection mirror surface on the side of the direction in which the reflection mirror is curved is of flat shape, deviation in relative support angle is generated in terms of working at the support portions of the opposite ends formed on the opposite ends of the reflection mirror in some cases. This is because the support portions of the opposite ends are flat. The reflection mirror is placed at each support portion with an angle along a ridgeline of each seat surface, but the angle of the seat surface of opposite ends is varied within a range of tolerances, the angles of reflection mirrors to be placed are different on the opposite ends. Therefore, the reflection mirror is twisted when the seat surface is flat and as a result, the optical characteristics are further deteriorated.